In recent years bipolar membranes have gained prominence as "water-splitters" and "acid-base" generators in electrodialysis operations. The main purpose of the bipolar membranes in such processes is to promote the dissociation of water into hydrogen and hydroxyl ions under the influence of an applied electrical potential. The commercial feasibility of all such processes requires the membranes to operate with low potential drops across them and with high current efficiencies at practical current densities and electrolyte concentrations.
Bipolar membranes consist of two oppositely charged ion exchange layers or membranes, either in close proximity or joined together. Each component is composed of a semi-permeable matrix material, for example, polyethylene, initimately containing a charge-bearing portion, for example, functionalized, cross-linked polystyrene. Usually strong acidic (sulfonic acid) and strong basic (quaternary ammonium) groups are introduced into the separate membranes. Each component is capable of transporting ions of opposite charge and hence also conducting an electric current. Under the influence of a direct current, cations or hydrogen ions present in the cationic-permeable component migrate toward the cathode and anions or hydroxyl ions in the anion-permeable component migrate toward the anode. This soon creates a depletion of ions in the vicinity of the interface and continuation of the flow of current is maintained only by the dissociation of water into hydrogen ions and hydroxyl ions. In order that the process is sustained, water must diffuse into the membrane to replenish that consumed by the dissociation.
The potential drop across the bipolar membrane is the sum of the individual potential drops of each component and the interface plus that required to split water (E.sub.m H.sup.+,OH.sup.- .apprxeq.0.8 v) E.sub.m =E anionic+E cationic+E interface+E.sub.m H.sup.+,OH.sup.- The component membrane potentials may be minimized by increasing their exchange capacities, while lowering interface potential drops depends upon the means one uses to join the components.
Individual component membranes of the type discussed here are subject of a patent by J. F. A. Hazenberg and B. F. Knol (U.S. Pat. No. 3,133,889, May 19, 1964). Their joining together into a bipolar membrane by a hydraulic press is disclosed in U.S. Pat. No. 3,372,101. Bipolar membranes so prepared however are found to possess relatively high potential drops across them, evidently produced by the dislocation of fixed charges during fusion and the creation of microthin areas of essentially pure polyethylene. To avoid this drawback various means have been attempted such as by adding ion exchange resin mixtures between the components prior to fusion to increase the charge concentration at the interface to avoid this difficulty. In so far as is known, membranes made in this manner have not yielded practically suitable products. In any event there is a need for a durable bipolar membrane with a relatively low potential drop which would be attractive in various chemical process operations such as, for example, the treatment of effluent streams e.g., removal of SO.sub.2 and the conversion of H.sub.2 SiF.sub.6 to HF.